In general, in mechanical systems having sliding portion, i.e. internal combustion engine, automatic transmission, damper, drive-train module such as power steering, gears, and so on, lubricating oil is used to lubricate the motions. Particularly, lubricating oil for internal combustion engine mainly lubricates piston ring and cylinder liner, bearings of crankshaft, connecting rods, valve train, but also works as coolant or detergent-and-dispersant of combustion product in the engine, further works as rust and corrosion inhibitor.
Hence, the lubricating oil for internal combustion engine is required various functions; in addition to this, in recent years, trade-off functions of lubricating oil, such as: improvement of fuel saving; minimization of ash-content, phosphorus-content, and sulfur-content to minimize the effect on exhaust-gas after-treatment device; improvement of long-drain performance; and so on, are also required at high degree of compatibility. In internal combustion engine, since energy loss at friction parts where the lubricating oil is involved with is large, in order to reduce friction loss and fuel consumption, for example, as disclosed in Patent Document 1, lubricating oil which is a combination of friction reducer and other various additives is used.
Conventionally, so as to lower friction coefficient of the lubricating oil, addition of organomolybdenum compound represented by molybdenum dithiocarbamate and molybdenum dithiophosphate has been carried out. In addition to this, blending of the organomolybdenum compound and metallic detergent (See e.g., Patent Document 2.), or blending of the organomolybdenum compound and sulfur compound (See e.g., Patent Document 3.) have also been carried out.
However, in diesel engine and direct-injection gasoline engine, large amount of soot produced within piston is mixed into the engine oil. This soot absorbs polar additive in the oil as it has surface activity, and it also scrapes off a coating formed on the friction surface. Thereby, under such a severe friction condition, even if the organomolybdenum compound which is supposed to be the most effective substance for reducing friction is used, sufficient friction reducing effect cannot be obtained because of the inhibition by the soot and abrasion powder of metal. Few studies to overcome this problem have been made; the only suggestion is to mix alkali metal borate salt hydrate (See e.g., Patent Document 4.) and the like for improving fuel saving performance of diesel engine.
On the other hand, lowering viscosity of lubricating oil is also known as an effective measure to improve fuel saving performance, and multigrade diesel engine oil, which is obtained by adding viscosity index improver like polymethacrylate and ethylene-propylene copolymer to low-viscosity lubricating oil, is generally used. Nevertheless, fuel saving effect of the multigrade diesel engine oil in which the above viscosity index improver is only added is little and it is far from sufficient. So, in the field of diesel engine and direct-injection gasoline engine, technical development of engine oil which enables to sufficiently attain the fuel saving effect has been required.
As for diesel engine, reduction of NOX and particulate matters (SPM) is an urgent need, in order to reduce the exhaust gas, introduction of means for reducing exhaust gas, for instance, high-pressure injection, Exhaust Gas Recirculation system (EGR), oxidation catalyst, Diesel Particulate Filter (DPF), or NOX storage reduction catalyst, are considered.
However, among the means for reducing exhaust gas, when particularly oxidation catalyst of exhaust-gas after-treatment device, NOx storage reduction catalyst, and DPF are used, it is known that life-time thereof is shortened by engine oil to be used depending on the composition. For example, when a lubricating oil containing zinc dialkyl dithiophosphate (hereinafter, refer to as “ZnDTP”.) which is effective as an anti-wear agent or an antioxidant (peroxide decomposer) is used, zinc atom in ZnDTP forms oxide, phosphate, sulfate, or the like during the combustion process; these substances are deposited on the catalytic surface or in the filter, and there is a possibility of spoiling detergent performance of the above exhaust-gas aftertreatment devices. As the countermeasure against this, it is preferable not to add ZnDTP to the lubricating oil for engine equipped with the above exhaust-gas after-treatment devices, or preferable to add the same as little as possible. Further, metallic detergent and sulfur content also tend to cause the above-mentioned problem since these are formed into metal oxide and sulfate and deposited as ash in the filters and the like, it is desirable to minimize the content thereof.
Moreover, because of the large amount of soot being mixed into the lubricating oil, there is a concern that diesel engine, specifically an EGR-equipped diesel engine might have increase of wear in valve train system and the like and deterioration of high-temperature detergency like piston detergency are worried about. Furthermore, there is another concern that direct-injection gasoline engine might have negative effect similar to the above by the soot being mixed into the lubricating oil and have combustion chamber deposit and valve deposit. Hence, simply reducing the content of ZnDTP, metallic detergent, and sulfur content is extremely difficult, it is necessary to consider a new means for make-up reduced detergency and anti-wear property involved in the reduction of these contents.
As a lubricating oil composition for engine equipped with an exhaust-gas after-treatment device, a diesel engine oil composition which minimize the sulfated ash content down to 0.7 mass % or less is proposed (See Patent Document 5.). Further, so as to improve detergency and anti-wear property under a condition of soot contamination, engine oil containing dispersant-type viscosity index improver is also proposed (See Patent Documents 6 and 7.). However, in these proposals, high-temperature detergency and property of base-number retention are not necessarily sufficient when metallic detergent is reduced; high-temperature detergency in case of reducing ZnDTP content and anti-wear property under a condition of soot contamination are not sufficiently considered. Accordingly, in order to maintain or improve the level of high-temperature detergency and property of base-number retention high and to inhibit soot-derived wear which is unignorable when ZnDTP content is reduced, furthermore research and development are required.
As a method for significantly enhancing high-temperature detergency, property of base-number retention and low friction of lubricating oil, lubricating oil composition obtained by blending a salicylate detergent of which ZnDTP content is reduced and of which the metal ratio is prepared to become 2.6 or less or salicylate detergent of which metal ratio is less than 2 with over-based sulfonate is commonly known. Nevertheless, anti-wear property under the condition of soot mixing is not necessarily sufficient, improvement of such a problem is required (See Patent Documents 8˜10.).
Therefore, in order to maintain or improve the level of high-temperature detergency and property of base-number retention high and to inhibit soot-derived wear which is unignorable when ZnDTP content is reduced, furthermore research and development are required.
Patent Document 1: Japanese Patent Application Examined No. 03-23595
Patent Document 2: Japanese Patent Application Examined No. 06-62983
Patent Document 3: Japanese Patent Application Examined No. 05-83599
Patent Document 4: Japanese Patent Application Examined No. 01-48319
Patent Document 5: Japanese Patent Application Laid-Open (JP-A) No. 2000-256690
Patent Document 6: JP-A No. 2001-279287
Patent Document 7: JP-A No. 2004-10799
Patent Document 8: Japanese Patent No. 3662228
Patent Document 9: Japanese Patent No. 3709379
Patent Document 10: Japanese Patent No. 3738228